(1) Field of the Invention
The present invention generally relates to a piston of an engine, and more particularly to a method of producing a piston of an internal combustion engine through casting.
(2) Description of the Related Art
A piston of an internal combustion engine generally has a piston head, a skirt, a pin boss, and a supporting area, the supporting area connecting a periphery of the pin boss with the skirt. The piston is usually produced from a light alloy, such as an aluminum alloy, through casting.
The piston has a complicated configuration, as mentioned above, and it is necessary that a plurality of molds be used by the casting for the production of the piston, and that the molds be arranged to form cavities corresponding to the complicated configuration of the piston. The molds must be arranged so as to form the configuration of the piston and facilitate the removal of the molds after the casting is finished.
For example, Japanese Laid-Open Patent Application No. 5-86971 discloses a method of producing a piston through casting. In this piston production method, a plurality of molds are arranged to form cavities corresponding to the configuration of the piston, and molten metal is poured into the cavities of the molds so that the casting for the production of the piston is performed. Japanese Laid-Open Patent Application No. 2-220733 discloses a method of producing a piston through casting. In this piston production method, a plurality of molds are arranged to form cavities corresponding to the configuration of the piston, and the casting for the production of the piston is performed. The molds of the above-mentioned publication include a fusible core mold supported by a strut, and the fusible core mold is arranged to form an internal cavity inside a top ring groove of the piston.
On the other hand, when the engine is assembled, a piston pin is fitted into the pin boss of the piston, and the piston is connected to a connecting rod by the piston pin. The connecting rod connects a crankshaft of the engine to the piston.
When the crankshaft of the engine is rotated, the piston moves up and down in a cylinder of the engine. At the time of the combustion stroke of the engine, the piston transmits the power, obtained by the combustion of fuel in the combustion chamber, to the crankshaft through the connecting rod and the piston pin. During the operation of the engine, compression and expansion forces from the connecting rod are exerted on the pin boss of the piston by the piston pin. These forces are transmitted from the pin boss to the supporting area, and a great stress on the supporting area of the piston is produced. Since the piston pin is supported on the pin boss of the piston, the corners of the supporting area around the periphery of the pin boss are subjected to the stress concentration.
In addition, the skirt of the piston is subjected to a frictional stress due to the reciprocating motion of the piston relative to the cylinder of the engine, and a great stress at a central portion of the skirt of the piston is produced.
As described above, when the piston is produced through the casting, the plurality of molds are arranged to form cavities corresponding to the configuration of the piston. When the casting for the production of the piston is performed, molten metal may flow out of a clearance between two of the molds which are contacted by each other, and a flash on a mold matching surface between the two molds may be produced as the result of the casting. The flash has sharp edges, and the stress concentration on the flash of the casting is very likely to take place.
For this reason, if the flash at any of the positions of the piston where a great stress is produced, such as the corners of the supporting area or the central portion of the skirt, is produced as a result of the casting, such positions of the piston may be easily cracked or damaged. In the conventional piston production method of the above publication, effective measures for preventing the occurrence of the flash at such positions of the piston as the result of the casting are not taken into account.